30
A closed return duct system must be used, with the return
duct connected to the furnace.
NOTE:
Ductwork must
never be attached to the back of the furnace. For upflow
installations requiring 1800 CFM or more, use either two
side returns or bottom return or a combination of side /
bottom. Flexible joints may be used for supply and return
connections to reduce noise transmission. To prevent the
blower from interfering with combustion air or draft when a
central return is used, a connecting duct must be installed
between the unit and the utility room wall. Never use a room,
closet, or alcove as a return air chamber.
Checking Duct Static
WARNING
Edges of sheet metal holes may be sharp. Use gloves as
a precaution when removing sheet metal from return air
openings.
Refer to your furnace rating plate for the maximum ESP
(external duct static) rating.
Total external static refers to everything external to the
furnace cabinet. Cooling coils, filters, ducts, grilles, registers
must all be considered when reading your total external static
pressure. The supply duct pressure must be read between
the furnace and the cooling coil. This reading is usually taken
by removing the “A” shaped block off plate from the end on
the coil; drilling a test hole in it and reinstalling the block off
plate. Take a duct static reading at the test hole. Tape up the
test hole after your test is complete. The negative pressure
must be read between the filter and the furnace blower.
Too much external static pressure will result in insufficient air
that can cause excessive temperature rise. This can cause
limit switch tripping and heat exchanger failure.
To determine total external duct static pressure, proceed as
follows:
1. With clean filters in the furnace, use a draft gauge
(inclined manometer) to measure the static pressure
of the return duct at the inlet of the furnace. (Negative
Pressure)
2. Measure the static pressure of the supply duct.
(Positive Pressure)
3. The difference between the two numbers is .4” w.c.
Example:
static reading from return duct = -.1” w.c.
static reading from supply duct = .3” w.c.
total external static pressure on this system = .4” w.c.
NOTE:
Both readings may be taken simultaneously and read
directly on the manometer if so desired. If an air conditioner
coil or Electronic Air Cleaner is used in conjunction with the
furnace, the readings must also include theses components,
as shown in the following drawing.
4. Consult proper tables for the quantity of air.
If the total external static pressure exceeds the maximum
listed on the furnace rating plate, check for closed dampers,
registers, undersized and/or oversized poorly laid out duct
work.
The temperature rise of the furnace must be within the
temperature rise range listed on the furnace rating plate.
Bottom Return Air Opening [Upflow Models]
The bottom return air opening on upflow models utilizes a
“lance and cut” method to remove sheet metal from the duct
opening in the base pan. To remove, simply press out the
lanced sections by hand to expose the metal strips retaining
the sheet metal over the duct opening. Using tin snips, cut
the metal strips and remove the sheet metal covering the
duct opening. In the corners of the opening, cut the sheet
metal along the scribe lines to free the duct flanges. Using
the scribe line along the duct flange as a guide, unfold the
duct flanges around the perimeter of the opening using a
pair of seamer pliers or seamer tongs.
NOTE:
Airflow area
will be reduced by approximately 18% if duct flanges are not
unfolded. This could cause performance issues and noise
issues.
CUT FOUR CORNERS
AFTER REMOVING SHEET
METAL
CUT USING TIN SNIPS
PRESS OUT BY HAND
SCRIBE LINES OUTLINING
DUCT FLANGES
Duct Flange Cut Outs
Figure 37
When the furnace is used in connection with a cooling unit,
the furnace should be installed in parallel with or on the
upstream side of the cooling unit to avoid condensation in
the heating element. With a parallel flow arrangement, the
dampers or other means used to control the flow of air must
be adequate to prevent chilled air from entering the furnace
and, if manually operated, must be equipped with means to
prevent operation of either unit unless the damper is in the
full heat or cool position.
When the furnace is installed without a cooling coil, it is
recommended that a removable access panel be provided
in the outlet air duct. This opening shall be accessible when
the furnace is installed and shall be of such a size that the
heat exchanger can be viewed for visual light inspection
or such that a sampling probe can be inserted into the
airstream. The access panel must be made to prevent air
leaks when the furnace is in operation.
